1. Field of the Invention
This invention relates to liquid ink compositions, and particularly to liquid ink compositions that are useful in printing processes. In particular, this invention relates to a liquid ink which exhibits improved dispersion stability, improved blocking resistance, and improved chargeability when used in any imaging process, including but not limited to ink transfer processes, ionographic, ink jet, bubble jet, electrographic and electrophotographic color printing or proofing processes.
2. Background of the Art
Liquid inks are widely used in a variety of imaging and printing processes, for example offset, bubble jet, ink jet, intaglio, rotogravure, electrographic, and electrophotographic printing. Many characteristics that are desired in pigment dispersions for liquid inks are the same for each of the respective processes, even though the final ink formulations and process of application may be substantially different. For example, the stability of the pigment dispersion on the shelf, under shear conditions, and under high voltage fields is an important consideration regardless of the final use of the liquid ink. The art continuously searches for more stable pigment dispersions to provide more flexibility in ink formulations to provide better efficiency and waste reduction in the various printing processes.
In electrophotographic applications, which includes devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks also are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the photoconductor to radiation in an imagewise pattern, developing a temporary image on the photoconductor by contacting the photoconductor with a liquid developer, and finally transferring the temporary image to a receptor. The final transfer step may be performed either directly from the photoconductor or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
Liquid toners typically comprise an electrically insulating liquid that serves as a carrier for a dispersion of charged particles, known as toner particles. These toner particles are composed of at least a colorant (e.g., pigment or dye) and a polymeric binder. A charge control agent is often included as a component of the liquid developer to regulate the polarity and magnitude of the charge on the toner particles. Liquid toners can be categorized into two primary classes. For convenience, the two classes will be referred to as conventional liquid toners and organosol toners. Between these two classes of liquid toners, organosol toners are preferred in electrophotograph applications for their stability.
Stable organosols contain colloidal (approximately 0.1-1 micron diameter) particles of polymeric binder. The organosol particles are typically synthesized by nonaqueous dispersion polymerization in a low dielectric hydrocarbon solvent. These organosol particles are sterically-stabilized with respect to aggregation by the use of a physically-adsorbed or chemically-grafted soluble polymer. Details of the mechanism of such steric stabilization are provided in Napper, D. H., Polymeric Stabilization of Colloidal Dispersions, Academic Press, New York, N.Y., 1983. Procedures for effecting the synthesis of self-stable organosols are known to those skilled in the art and are described in Dispersion Polymerization in Organic Media, K. E. J. Barrett, ed., John Wiley: New York, N.Y., 1975.
The most commonly used non-aqueous dispersion polymerization method is a free radical polymerization carried out when one or more ethylenically-unsaturated (typically acrylic or methacrylic) monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed amphipathic polymer. The preformed amphipathic polymer, commonly referred to as the stabilizer, is comprised of two distinct units, one essentially insoluble in the hydrocarbon medium, the other freely soluble. When the polymerization process to manufacture the organosol particle proceeds to a fractional conversion of monomer corresponding to a critical molecular weight, the solubility limit of the polymer is exceeded and the polymer precipitates from solution, forming a “core” particle. The amphipathic polymer then either adsorbs onto, ionically bonds to or covalently bonds to the core, which core continues to grow as a discrete particle. The particles continue to grow until monomer is depleted, and the attached amphipathic polymer “shell” acts to sterically-stabilize the growing core particles with respect to aggregation. The resulting non-aqueous colloidal dispersion (organosol) comprises core/shell polymer particles with a number average diameter in the range of about 0.05-5 microns.
The resulting organosols can be subsequently converted to liquid toners by simple incorporation or mixing of the colorant (pigment) and a charge director, followed by high shear homogenization, ball-milling, attritor milling, high energy bead (sand) milling or other size reduction processes or mixing means known in the art for effecting particle size reduction in forming a dispersion. The input of mechanical energy to the dispersion during milling acts to break down pigment agglomerates into primary particles (e.g., from about 0.05-1.0 micron number average diameter) and to “shred” the organosol into fragments that adhere to the newly-created pigment surface, thereby acting to sterically-stabilize the pigment particles with respect to aggregation. The charge director may physically or chemically adsorb onto the pigment, the organosol or both. The result is a sterically-stabilized, charged, non-aqueous pigment dispersion having particles in the size range of about 0.05-5.0 microns number average diameter, with typical toner particle number average diameters between about 0.15-1.0 microns. Such a sterically-stabllized dispersion is ideally suited for use in high resolution printing.
A problem in formulating liquid inks is the tack of the image on the final receptor. If the image has a residual tack, then the image may become embossed or picked off when placed in contact with another surface. This phenomenon is called blocking. This is especially a problem when printed sheets are placed in a stack, as when printed sheets are fed out of a printer into a collector. If the image is tacky, it may adhere or transfer portions of the image to the backside of the adjacent sheet. To address this concern, a film laminate or protective layer is typically placed over the surface of the image. This adds both an extra cost of materials and extra process steps to apply the protective layer.
Another problem in formulating rapid self-fixing liquid inks is the difficulty in obtaining liquid inks that have both excellent aggregation stability and sedimentation stability on the shelf It is known in the art that film-forming liquid inks comprising stable organosols generally exhibit either excellent aggregation stability or sedimentation stability, but not necessarily both. The aggregation stability of an organosol is related to the tendency of the core/shell particles to aggregate into effectively larger groups of particles. The sedimentation stability of an organosol is related to the tendency of the components, especially the colorant particles, of the organosol to settle out of the dispersion or suspension. Hence, there is a need for liquid ink compositions having both excellent aggregation stability and sedimentation stability.
An important consideration in formulating liquid inks is the chargeability of the liquid ink. It is desirable for a liquid ink to have a high chargeability so that it can acquire enough quantity of charge to be forced under an electrical field to migrate and then to plate upon the imaged areas on the photoreceptor with differentiation based upon charge differences on the photoreceptor. The chargeability of a liquid ink is measured by its conductivity and its mobility. In general, liquid inks with a high ink conductivity and a high ink mobility are desirable.
This invention provides a stable organosol with a novel composition that exhibits increased dispersion stability, improved blocking resistance, and improved chargeability.